I have a very clear message for president Assad. It is time for him to go.

It is time for transition in the regime. If there isn't transition it's quite clear there's going to be civil war.

My message to (Russian) president (Vladimir) Putin is, it's time for the UN Security Council to pass clear and tough messages about sanctions.

Mr Cameron was commenting a day after rebels penetrated the heart of Syria's government elite, detonating a bomb inside a crisis meeting in Damascus that killed three leaders of the regime, including Assad's brother-in-law and the defence minister.

Mr Cameron added: "I don't think any regime that carries out acts as they have should survive."

As the Prime Minister made his comments, reports surfaced that new clashes had erupted near the government headquarters in Damascus after rebels attacked forces loyal to President Bashar al-Assad.

Activists and rebels say regime supporters have deployed armoured vehicles and increased roadblocks across the city.

At least one person was reported killed in the fighting in the Ikhlas neighbourhood adjacent to the Council of Ministers, a huge complex, and a Damascus University campus, they said.

Syria's President Bashar al-Assad.

Reports are also circulating that the Syrian president is now in the coastal city of Latakia, directing a response to the assassination of three of his top lieutenants.

It was not clear whether Assad travelled to the Mediterranean sea resort before or after the attack.